Optical memory technology that uses optical disks as high-density, large-volume memory media gradually is being applied widely to and entering general use in digital audio disks, video disks, document file disks and also data files. To successfully achieve recording onto and reproduction of information from an optical disk with high reliability via a minutely narrowed light beam, there is a need for a focusing function forming a minute spot at the diffraction limit, focus control and tracking control of the optical system, and a pit signal (“information signal”) detection function.
With recent advances in optical system design technology and the shortening of wavelengths of the semiconductor lasers serving as light sources, the development of optical disks containing volumes of memory at greater than conventional densities is progressing. As an approach to higher densities, increasing the optical disk side numerical aperture (NA) of a focusing optical system that minutely stops down a light beam onto an optical disk has been investigated. A problem that occurs at this time is that there is an increase in aberration caused by an inclination of the disk in relation to the light axis (what is known as “tilt”). When the NA is made large, the aberration caused by tilt increases. It is possible to prevent this by reducing the thickness (substrate thickness) of the transparent substrate of the optical disk.
A Compact Disc (CD), which can be considered a first generation optical disk, is used with a light source emitting infrared light (a wavelength λ3 is 780 nm to 820 nm) and an objective lens with an NA of 0.45, and has a substrate thickness of approximately 1.2 mm. A Digital Versatile Disc (DVD), which can be considered a second generation optical disk, is used with a light source emitting red light (a wavelength λ2 is 630 nm to 680 nm) and an objective lens with an NA of 0.6, and has a substrate thickness of approximately 0.6 mm. And, a system has been proposed in which a third generation optical disk is used with a light source that emits blue light (a wavelength λ1 is 380 nm to 420 nm) and an objective lens with an NA of 0.85, the disk having a substrate thickness of 0.1 mm.
It should be noted that in this specification, the substrate thickness means the thickness of the transparent substrate from the face at which a light beam is incident on the optical disk (or optical recording medium) to the information recording surface.
Thus, the thickness of the substrate of optical disks becomes thinner with increasing recording density. From the standpoint of economics and the space occupied by the device, it is desirable that a single optical information recording and reproduction apparatus is capable of recording and reproducing optical disks of different substrate thickness and recording density. For this purpose, there is a need for an optical head device that is provided with a focusing optical system that is capable of focusing a light beam up to the diffraction limit onto optical disks of different substrate thicknesses.
An example of a device that records and reproduces information from both DVD and CD optical disks (information recording media) is proposed in the Patent Document 1 described below. As a first conventional example, this content is described simply using FIGS. 58 to 60. FIG. 58 is a structural overview of an optical head 300. FIG. 58A shows the manner in which information is recorded onto or reproduced from a DVD and FIG. 58B shows the manner in which information is recorded onto or reproduced from a CD. It contains a red semiconductor laser 301 that emits light of a wavelength of 635 nm to 650 nm, and an infrared semiconductor laser 302 that emits light of a wavelength of 780 nm.
When reproducing a DVD 308, which is a second information recording medium, the light emitted from the red semiconductor laser 301 passes through a wavelength selecting prism 303, and is converted to collimated light by a collimator lens 304. The light that was converted to collimated light is reflected by a beam splitter 305, passes through a dichroic hologram 306, is converted to convergent light by an objective lens 307, and is irradiated onto the DVD 308. The light that was reflected by the DVD 308 again passes through the objective lens 307 and the dichroic hologram 306, passes through the beam splitter 305, is converted to convergent light by a detecting lens 309, and is focused onto a photodetector 310.
When reproducing a CD 311, which is a third information recording medium, the light emitted from the infrared semiconductor laser 302 is reflected by the wavelength selecting prism 303, and is converted to collimated light by a collimator lens 304. The light that was converted to collimated light is reflected by a beam splitter 305, is diffracted by the dichroic hologram 306, is converted to convergent light by an objective lens 307, and is irradiated onto the CD 311. The light that was reflected by the CD 311 again passes through the objective lens 307 and the dichroic hologram 306, passes through the beam splitter 305, is converted to convergent light by the detecting lens 309, and is focused onto the photodetector 310.
Spherical aberration caused by the difference in substrate thickness of DVDs and CDs is corrected by the dichroic hologram 306. FIG. 59 is a cross-sectional view of the dichroic hologram 306. Grooves of depth d, 2d and 3d are arranged in that order on the surface of the dichroic hologram 306. The depth d is determined such that,d =λ1/(n1−1)where λ1 is the wavelength of the red semiconductor laser and n1 is the refractive index of the dichroic hologram 306 at the wavelength λ1. In this way, the transmittance of the light of wavelength λ1, increases without diffracting the light.
Here, the wavelength of light emitted from the infrared semiconductor laser is λ2, and the refractive index of the dichroic hologram 306 at the wavelength λ2 is n2. FIG. 60A shows the wavefront after the light of wavelength λ2 has passed the dichroic hologram 306, in which,d×(n2−1)/λ2=0.75.In this case, a phase shift of 0.75 times the wavelength occurs per step. As phase shifts of greater than one can be ignored, FIG. 60B shows a wavefront that is re-written, based only on that portion to the right of the decimal point. This wavefront becomes first order diffraction light, which has a high diffraction efficiency at one side.
Furthermore, in the non-Patent Document 1 described below an example is given of a device for reproducing information on CDs, DVDs and ultra high density optical disks. This is briefly explained using FIGS. 61 and 62 as a second conventional example. FIG. 61 is a structural overview showing an optical head.
Collimated light emitted from an optical system 201 that contains a blue light source of wavelength λ1=405 nm passes through prisms 204, 205 and a phase plate 206, which will be explained later, is focused by an objective lens 207, and is irradiated onto an information recording surface of an optical disk 208 (an ultra high density optical disk) whose substrate thickness is 0.1 mm.
The light that was reflected by the optical disk 208 returns back along the travel path and is detected by a photodetector of the optical system 201. The diverging light that is emitted by an optical system 202 that contains a source of red light of wavelength λ2=650 nm is reflected by the prism 204, passes through the prism 205 and the phase plate 206, is focused by the objective lens 207 and is irradiated onto an information recording surface of an optical disk 209 (DVD), whose substrate thickness is 0.6 mm.
The light that was reflected from the optical disk 209 returns back along the travel path, and is detected by a photodetector of the optical system 202. The diverging light emitted by an optical system 203, which has a source of infrared light of a wavelength λ3=780 nm is reflected by the prism 205, passes through the phase plate 206, is focused by the objective lens 207, and is irradiated onto an information recording surface of an optical disk 210 (CD), whose substrate thickness is 1.2 mm. The light that was reflected by the optical disk 210 returns back along the travel path, and is detected by a photodetector of the optical system 203.
The objective lens 207 is designed so as to handle substrate thicknesses of 0.1 mm, and spherical aberration occurs in CDs and DVDs because of the difference in substrate thickness. Correction of this spherical aberration occurs due to the degree of divergence of the diverging light that is emitted by the optical system 202 and optical system 203, and due to the phase plate 206. Different spherical aberration is generated when divergent light is incident on the objective lens, so it is possible to cancel out spherical aberration caused by the difference in substrate thickness by this new spherical aberration.
The degree of divergence of the diverging light is set such that spherical aberration is a minimum. Spherical aberration caused by the diverging light cannot be completely corrected, and higher order spherical aberrations (principally fifth order spherical aberrations) remain. These fifth order spherical aberrations are corrected by the phase plate 206.
FIG. 62 shows a surface (FIG. 62A) and a lateral view (FIG. 62B) of the phase plate 206. If the refractive index at the wavelength λ1 is defined as n1, and h=λ1/(n1−1), then the phase plate 206 is constituted by phase shift steps 206a of height h and height 3h. The height h generates a phase shift of 1λ (where λ is the wavelength that is used) in the light of wavelength λ1, however this does not affect the phase distribution and there is no impediment to recording or reproduction of the optical disk 208.
On the other hand, if the refractive index of the phase plate 206 at the wavelength λ2 is n2, then a phase shift of the light of wavelength λ2 of h/λ2×(n2−1)=0.625 λ is generated. Furthermore, if the refractive index of the phase plate 206 at the wavelength λ3 is n3, then a phase shift of the light of wavelength λ3 of h/λ3×(n3−1)=0.52 λ is generated. In relation to DVDs and CDs, this wave shift is used to convert the wavefronts, and the remaining fifth order spherical aberrations are corrected.
Moreover, the Patent Document 2 described below proposes a method for reproducing information using an objective lens that is capable of recording and reproducing ultra high density optical disks, and two objective lenses that are capable of reproducing CDs and DVDs. This is described briefly as a third conventional example, using FIG. 63.
A lens holder 233 is provided with an objective lens 231 that is used when recording onto and replaying from ultra high density optical disks, an objective lens 232 that is used when reproducing DVDs and CDs, and drive coils 234, and is suspended by wires 236 from a fixed portion 237.
A magnetic circuit is constituted by a magnet 238 and a yoke 239. An electromagnetic force is caused by the flow of electric current through the drive coil 234, and the objective lenses 231 and 232 are driven in the focusing direction and the tracking direction. In the third conventional example, which of the objective lenses 231 and 232 is used depends on the optical disk to be recorded and reproduced.
Furthermore, as a technique for correcting chromatic aberration, a chromatic aberration correcting hologram is proposed in the Patent Document 3 described below, in which the cross-sectional shape of the optical element is saw tooth shaped, wherein light of a first wavelength λ1 is corrected using second order diffracted light, and light of a second wavelength λ2 is corrected using first order diffracted light.
However, in the optical head of the first conventional example, when light is irradiated onto optical disks that have widely different substrate thicknesses, such as a substrate thickness of 1.2 mm and a substrate thickness of 0.1 mm, there is the problem that the distance between the disk and the objective lens changes significantly, the movable range of the actuator increases, and the head becomes large. Moreover, there is the problem that in order to detect the light that corresponds to the three types of light sources, the number of signal wires increases and the width of the flexible cable that connects the optical head and the optical disk drive is wider.
Furthermore, in the optical disk device according to the second conventional example, since the light is incident on the objective lens as divergent light when reproducing CDs and DVDs, there is the problem that when the objective lens is driven in the tracking direction, a large coma aberration is generated and the optical disks cannot be favorably reproduced.
Furthermore, in the optical disk device of the third conventional example, because the objective lenses 231 and 232 are lined up in a tangential direction (y direction) and the objective lens 231 is arranged such that it is positioned on a straight line in the tracking direction (x direction) that passes through a rotational center O of the optical disk, there is the problem that DVDs and CDs that use the objective lens 232 cannot use the differential push-pull (DPP) method or the three beam method, which are common tracking detection methods. This problem is described using FIG. 64. The DPP method or the three beam method use a main spot for reproduction, and two sub spots for tracking detection. A main spot 232a of the objective lens 232 shown in FIG. 63 is in a spot position 150a shown in FIG. 64. The subspots are in positions 150b and 150c, and are set at an optimal angle θ0 with respect to a reproduction track 153.
The spots move in the x-direction in accordance with the seek operation of the optical head, and the spot positions change to 151a, 151b and 151c. Because the spot positions 150a and 151a are not on the straight line that passes through the axis of rotation O of the optical disks in the x-direction, the angle θ0 changes to θ1 due to the seek operation of the optical head. That is to say, in the configuration of the third conventional example, there is the problem that tracking control cannot be carried out reliably.    Patent Document 1    JP H9-306018A    Patent Document 2    JP H11-120587A    Patent Document 3    JP 2001-60336    Non-Patent Document 1    Session We-C-05 of ISOM 2001 (p30 of the proceedings)